One transistor SOI non-volatile random access memory cell

ABSTRACT

One aspect of the present subject matter relates to a memory cell, or more specifically, to a one-transistor SOI non-volatile memory cell. In various embodiments, the memory cell includes a substrate, a buried insulator layer formed on the substrate, and a transistor formed on the buried insulator layer. The transistor includes a floating body region that includes a charge trapping material. A memory state of the memory cell is determined by trapped charges or neutralized charges in the charge trapping material. The transistor further includes a first diffusion region and a second diffusion region to provide a channel region in the body region between the first diffusion region and the second diffusion region. The transistor further includes a gate insulator layer formed over the channel region, and a gate formed over the gate insulator layer. Other aspects are provided herein.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. Ser. No. 10/232,846,filed on Aug. 30, 2002, which is incorporated herein by reference in itsentirety.

This application is related to the following commonly assigned U.S.patent applications which are herein incorporated by reference in theirentirety: “Scalable Flash/NV Structures & Devices With EnhancedEndurance,” U.S. application Ser. No. 09/944,985, filed on Aug. 30,2001; “Stable PD-SOI Devices and Methods,” U.S. application Ser. No.10/197,978, filed on Jul. 18, 2002; “Gated Lateral Thyristor-BasedRandom Access Memory Cell (GLTRAM),” U.S. application Ser. No.10/232,855, filed on Aug. 30, 2002; and “One-Device Non-Volatile RandomAccess Memory Cell,” U.S. application Ser. No. 10/232,848, filed on Aug.30, 2002.

TECHNICAL FIELD

This disclosure relates generally to integrated circuits, and moreparticularly, to non-volatile, silicon-on-insulator (SOI) memory cells.

BACKGROUND

Known dynamic random access memory (DRAM) devices include a switchingtransistor and an integrated storage capacitor tied to the storage nodeof the transistor. Incorporating a stacked capacitor or a trenchcapacitor in parallel with the depletion capacitance of the floatingstorage node enhances charge storage. Due to a finite charge leakageacross the depletion layer, the capacitor is frequently recharged orrefreshed to ensure data integrity in the DRAM device. Thus, such a DRAMdevice is volatile. A power failure causes permanent data loss in a DRAMdevice. DRAM devices are relatively inexpensive, power efficient, andfast compared to non-volatile random access memory (NVRAM) devices.

A minimum capacitance per cell is required to sense a conventional DRAMcell. A significant challenge for every succeeding generation of reducedfeature size is to provide this minimum capacitance per cell. A memorycell design goal is to achieve an 8F² DRAM cell. To that end, complexthree-dimensional capacitor structures have been designed. However,these complex three-dimensional capacitor structures are difficult tomanufacture and adversely impact yield. There has been serious concernof the scalability of the conventional DRAM cell beyond the 0.1 μmlithographic generation. The scaling problems have been aggravated byincreased device short channel effects and leakages associated withcomplicated capacitor structures. Thus, the elimination of the stackedcapacitor or trench capacitor in a DRAM cell is desirable.

A silicon-on-insulator (SOI) capacitor-less single-transistor DRAM cellhas been proposed by S. Okhonin et al. The state of the floating bodycharge in the transistor affects the channel conductance of thetransistor and defines the memory state (“1” or “0”) of the cell. Twomethods for generating carriers in the body were proposed. The generatedcarriers are holes for the partially depleted (PD) SOI-NFET or electronsfor the PD-SOI-PFET. One proposed method generates carriers using thedrain-edge high field effect associated with impact ionization. Inanother case, the carriers are generated by the parasitic bipolarphenomenon.

The memory retention for these SOI capacitor-less single-transistor DRAMcells depends on the device channel length. That is, the stored chargeretention time decreases with decreasing channel length. Additionally,the memory retention depends on recombination charge constants andmultiple recombination mechanisms, and thus is expected to be bothtemperature and process sensitive. Therefore, controlling the memoryretention between refresh operations is expected to be difficult.

Known non-volatile random access memory (NVRAM), such as Flash, EPROM,EEPROM, etc., store charge using a floating gate or a floating plate.Charge trapping centers and associated potential wells are created byforming nano-particles of metals or semiconductors in a large band gapinsulating matrix, or by forming nano-layers of metal, semiconductor ora small band gap insulator that interface with one or more large bandgap insulating layers. The floating plate or gate can be formed as anintegral part of the gate insulator stack of the switching transistor.

Field emission across the surrounding insulator causes the stored chargeto leak. The stored charge leakage from the floating plate or floatinggate is negligible for non-volatile memory devices because of the highband gap insulator. For example, silicon dioxide (SiO₂) has a 9 ev bandgap, and oxide-nitride-oxide (ONO) and other insulators have a band gapin the range of 4.5 ev to 9 ev. Thus, the memory device retains storeddata throughout a device's lifetime.

However, there are problems associated with NVRAM devices. The writingprocess, also referred to as “write-erase programming,” for non-volatilememory is slow and energy inefficient, and requires complex high voltagecircuitry for generating and routing high voltage. Additionally, thewrite-erase programming for non-volatile memory involves high-fieldphenomena (hot carrier or field emission) that degrades the surroundinginsulator. The degradation of the insulator eventually causessignificant leakage of the stored charge. Thus, the high-field phenomenanegatively affects the endurance (the number of write/erase cycles) ofthe NVRAM devices. The number of cycles of writing and erasing istypically limited to 1E6 cycles. Therefore, the available applicationsfor these known NVRAM devices is limited.

Floating plate non-volatile memory devices have been designed that use agate insulator stack with silicon-rich insulators. In these devices,injected charges (electrons or holes) are trapped and retained in localquantum wells provided by nano-particles of silicon embedded in a matrixof a high band gap insulator (also referred to as a “trapless” or“limited trap” insulator) such as silicon dioxide (SiO₂) or siliconnitride (Si₃N₄). In addition to silicon trapping centers, other trappingcenters include tungsten particles embedded in SiO₂, gold particlesembedded in SiO₂, and a tungsten oxide layer embedded in SiO₂.

There is a need in the art to provide dense and high speedcapacitor-less memory cells with data non-volatility similar to Flashdevices and DRAM-like endurance as provided by the present subjectmatter.

SUMMARY

The above mentioned problems are addressed by the present subject matterand will be understood by reading and studying the followingspecification. The present subject matter relates to non-volatile memorycells. In various embodiments, the memory cells are formed using onetransistor. In various embodiments, the memory cell transistor is apartially-depleted SOI field effect transistor (PD-SOI-FET) transistorwith a floating body that contains charge traps. In various embodiments,the memory cell transistor is a fully-depleted SOI field effecttransistor (FD-SOI-FET) transistor with a floating body that containscharge traps.

The present subject matter provides a binary memory state by trappingcharges in the floating body to provide a first state and byneutralizing and/or de-trapping the trapped charges in the floating bodyto provide a second state. Both states are stable to providenon-volatility. Various embodiments provide a charge trapping region inthe body of the transistor near the interface between the transistorbody and the buried insulator, such as buried oxide (BOX). Variousembodiments provide a charge trapping layer, such as asilicon-rich-nitride (SRN) layer, a silicon nitride or an oxynitride, ator near the BOX-body interface. The charges are neutralized by providingcharges of opposite polarity into the transistor body. Charge retentionis independent with respect to the device body length. The memory cellof the present subject matter is capable of long charge retention andnon-volatility. Additionally, the memory cell of the present subjectmatter provides high density (4F²) and fast DRAM-like read-writecapabilities.

One aspect of the present subject matter relates to a memory cell. Invarious embodiments, the memory cell includes a substrate, a buriedinsulator layer formed on the substrate, and a transistor formed on theburied insulator layer. The transistor includes a floating body regionthat includes a charge trapping material (or charge trapping region). Amemory state of the memory cell is determined by trapped charges orneutralized charges in the charge trapping material. The transistorfurther includes a first diffusion region and a second diffusion regionto provide a channel region in the body region between the firstdiffusion region and the second diffusion region. The transistor furtherincludes a gate insulator layer formed over the channel region, and agate formed over the gate insulator layer.

These and other aspects, embodiments, advantages, and features willbecome apparent from the following description of the present subjectmatter and the referenced drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an n-channel one transistor SOI non-volatile memorycell according to various embodiments of the present subject matter.

FIG. 2 illustrates a p-channel one transistor SOI non-volatile memorycell according to various embodiments of the present subject matter.

FIG. 3 illustrates a first memory read scheme according to variousembodiments of the present subject matter.

FIG. 4 illustrates a second memory read scheme according to variousembodiments of the present subject matter.

FIG. 5 illustrates electrical waveforms associated with reading a memorystate “1” and a memory state “0” according to various embodiments of thepresent subject matter.

FIGS. 6A-6D illustrate a write operation for a memory cell in a FET modeof operation according to various embodiments of the present subjectmatter.

FIGS. 7A-7B illustrate an erase operation for a memory cell in a FETmode of operation according to various embodiments of the presentsubject matter.

FIG. 8 illustrates electrical waveforms associated with writing anderasing a memory cell in a FET mode of operation according to variousembodiments of the present subject matter.

FIGS. 9A-9B illustrate a lateral parasitic bipolar junction transistor(BJT) associated with a FET device in the memory cell according tovarious embodiments of the present subject matter.

FIGS. 10A-10D illustrate a write operation for a memory cell in aparasitic BJT mode of operation according to various embodiments of thepresent subject matter.

FIGS. 11A-11B illustrate an erase operation for a memory cell in aparasitic BJT mode of operation according to various embodiments of thepresent subject matter.

FIG. 12 illustrates electrical waveforms associated with writing anderasing a memory cell in a parasitic bipolar mode of operation accordingto various embodiments of the present subject matter.

FIG. 13 is a simplified block diagram of a high-level organization ofvarious embodiments of a memory device according to various embodimentsof the present subject matter.

FIG. 14 is a simplified block diagram of a high-level organization ofvarious embodiments of an electronic system according to the presentsubject matter.

FIG. 15 is a graph showing refractive index of silicon-rich siliconnitride films versus SiH₂Cl₂/NH₃ flow rate ratio.

FIG. 16 is a graph showing current density versus applied field forsilicon-rich silicon nitride films having different percentages ofexcess silicon.

FIG. 17 is a graph showing flat band shift versus time at an appliedfield of 4×10⁶ volts/cm for silicon-rich silicon nitride films havingvarying percentages of excess silicon.

FIG. 18 is a graph showing flat band shift versus time at an appliedfield of 7×10⁶ volts/cm for silicon-rich silicon nitride films havingvarying percentages of excess silicon.

FIG. 19 is a graph showing apparent dielectric constant K versusrefractive index for both Silicon Rich Nitride (SRN) and Silicon RichOxide (SRO).

FIG. 20 is a graph showing a linear correlation between the ratio ofnitrogen over nitrogen plus oxygen in oxynitride films and therefractive index of these films.

FIG. 21 is a graph showing the etch rate of 7:1 buffered HF as afunction of refractive index.

FIG. 22 is a graph showing low field conduction of silicon-oxynitridefilms as a function of refractive index in a current range.

FIG. 23 is a graph illustrating current density versus field of Si₃N₄and Si_(x)O_(y)N_(z) and showing charge transport for the entire fieldrange for nitride, Si₂ON₂ oxynitride, and oxygen-rich oxynitride.

FIGS. 24A-24C illustrate a one transistor SOI non-volatile memory cellwith a sidewall charge trapping region according to various embodimentsof the present subject matter.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingswhich show, by way of illustration, specific aspects and embodiments inwhich the present subject matter may be practiced. These embodiments aredescribed in sufficient detail to enable those skilled in the art topractice the present subject matter. The various embodiments of thepresent subject matter are not necessarily mutually exclusive. Otherembodiments may be utilized and structural, logical, and electricalchanges may be made without departing from the scope of the presentsubject matter. In the following description, the terms wafer andsubstrate are interchangeably used to refer generally to any structureon which integrated circuits are formed, and also to such structuresduring various stages of integrated circuit fabrication. Both termsinclude doped and undoped semiconductors, epitaxial layers of asemiconductor on a supporting semiconductor or insulating material,combinations of such layers, as well as other such structures that areknown in the art. The term “horizontal” as used in this application isdefined as a plane parallel to the conventional plane or surface of awafer or substrate, regardless of the orientation of the wafer orsubstrate. The term “vertical” refers to a direction perpendicular tothe horizontal as defined above. Prepositions, such as “on”, “side” (asin sidewall), “higher”, “lower”, “over” and “under” are defined withrespect to the conventional plane or surface being on the top surface ofthe wafer or substrate, regardless of the orientation of the wafer orsubstrate. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present invention isdefined only by the appended claims, along with the full scope ofequivalents to which such claims are entitled.

The present subject matter relates to a one transistor, non-volatilememory cell. The memory cell is formed using silicon-on-insulator (SOI)technology. In various embodiments, the memory cell transistor is apartially-depleted SOI field effect transistor (PD-SOI-FET) with afloating body that contains charge traps. In various embodiments, thememory cell transistor is a fully-depleted SOI field effect transistor(FD-SOI-FET) with a floating body that contains charge traps. However,various embodiments of the present subject matter include other floatingbody transistors.

The one transistor SOI memory cell of the present subject matterachieves high density (4F²), has fast DRAM-like read/write capabilities,and has high-retention and non-volatility. A binary yet stable memorystate is provided by trapping charges in the floating body of the PD-SOItransistor, and by neutralizing (or discharging) the charges trapped inthe floating body. In various embodiments, the trapped charge isneutralized by injecting charge of opposite polarity into the body. Thestate of the memory cell is read by sensing the channel conductance ofthe cell transistor to determine if the cell transistor is in a chargedstate or a neutralized state, which can be defined as a logic or memorystate “1”, and a logic or memory state “0”. For example, the memory cellstate is determined by sensing the change in the device current that isassociated with the trapped stored-charge.

The present subject matter generates carriers in a floating body of thePD-SOI transistor, and traps the carriers in the floating body usingcharge traps. The binary memory state is provided by trapping charges inthe floating body and by neutralizing the trapped charge in the floatingbody. In various embodiments, the charge traps are provided by a chargetrapping layer in the floating body. According to various embodiments,the charge trapping layer includes silicon-rich-nitride (SRN). Thetrapped carriers are neutralized by generating and injecting charges ofopposite polarity. The concept is also applicable to FD-SOI transistors.Additionally, the trapping of carriers could also be achieved in anyregion of the floating body other than in the immediate vicinity of thechannel region. For example, in various embodiments, the charge trappingregions are formed along one or more of the body sidewalls. An exampleof a sidewall charge trapping region is shown in and described withrespect to FIGS. 24A-24C.

According to various embodiments, the memory cell provides an energybarrier for the stored charge in the order of 1 ev or less. Thus, forvarious embodiments, the memory cell is capable of having long chargeretention for both the charged state and the neutralized state. Thecharge retention is independent of the channel length. This long chargeretention provides the memory cell with a non-volatile characteristic.The degree of non-volatility can be altered by altering the trappingmaterial and thereby modifying the energy barrier (trapped energydepth). Therefore, various embodiments have an appropriate trappingmaterial to provide a non-volatile random access memory, and variousembodiments have an appropriate trapping material to provide anon-volatile write once, read only memory.

Those of ordinary skill in the art will appreciate, upon reading andunderstanding this disclosure, that the present subject matter providesa number of benefits. These benefits include inexpensive and densememories. The memory cell (4F²) of the present invention is twice asdense as a conventional DRAM (8F²). Another benefit is non-volatility,thus eliminating the need to refresh the state of the memory cell.Another benefit of the present subject matter is that the memory cell ofthe present subject matter is energy efficient. Another benefit is thatthe present subject matter provides DRAM-like endurance within anon-volatile memory cell because the non-volatile memory cell of thepresent subject matter is capable of undergoing a large number ofwrite/erase cycles.

Memory Cell Structure

FIG. 1 illustrates an n-channel one transistor SOI non-volatile memorycell according to various embodiments of the present subject matter. Thememory cell 100 is formed on a substrate 102, such as a siliconsubstrate, for example. The illustration includes a substrate contact104 to contact the substrate 102. The memory cell 100 is isolated fromthe substrate 102 via a buried insulator, such as a buried oxide (BOX)layer 106, and from other devices via shallow trench isolation (STI)regions 108.

A PD-SOI NFET 110 is illustrated. The transistor 110 includes a floatingbody region 112, a first diffusion region 114, and a second diffusionregion 116. A channel region 118 is formed in the body region 112between the first and second diffusion regions 114 and 116. With respectto the illustrated n-channel FET, the body region 112 is doped with p−type impurities, and the first and second diffusion regions 114 and 116are doped with n+ impurities. The illustrated memory cell 100 includes abit line contact or drain contact 120 connected to the first diffusionregion 114, and a source line contact 122 connected to the seconddiffusion region 116. A gate 124, such as a polysilicon gate, isseparated from the channel region 118 by a gate insulator region 126.The illustrated memory cell 100 includes a word line contact or gatecontact 128 connected to the gate 124.

Unlike conventional FET devices, the body region 112 of the illustratedFET device includes a charge trapping region 130. Relatively simplefabrication techniques can be used to incorporate the charge trappingregion in the body region. However, as one of ordinary skill in the artwill understand upon reading and comprehending this disclosure, theincorporation of the charge trapping region 130 significantly improvesscalability and functionality without complex fabrication techniques.

The location of the charge trapping region 130 in the body region 112can be varied. In various embodiments, the location the charge trappingregion 130 is on or near the BOX-body interface. In other embodiments,the charge trapping region 130 is located elsewhere in the body region112, including the sidewall of the body region, at a sufficient depthsuch that it will not interfere with conductance. For example, variousembodiments of the present subject matter position the charge trappingregion 130 in the body region 112 at a depth below 200-300 Å (20-30 nm)from the surface where the charge flows.

The charge trapping region 130 provides localized quantum wells that areinitially neutral. These neutral wells attract charges and maintain thecharge species. Thus, charge traps are distinguished from recombinationcenters, which have been proposed in a body region to assist with therecombination of charges. Unlike the charge trapping regions, arecombination center provides a charged localized quantum well. Thecharged well attracts opposite charges which recombine to facilitatecharge neutrality.

One of ordinary skill in the art will understand, upon reading andcomprehending this disclosure, that the charge trapping region iscapable of being tailored to provide the device with desiredcharacteristics. For example, various embodiments of the present subjectmatter are designed to repeatedly trap and de-trap charges in the chargetrapping region so as to form a non-volatile random access memory.Various embodiments provide a charge trapping region with deep traps,and are designed to form a non-volatile, write once, read only memory.

In various embodiments, the charge trapping function of the chargetrapping region 130 is provided by a charge trapping layer. According tovarious embodiments, the charge trapping layer includes asilicon-rich-insulator (SRI) layer, such as a silicon-rich-nitride (SRN)or silicon-rich-oxide (SRO) layer, for example. SRI, SRN and SRO aredescribed with respect to FIGS. 15-19 below in the section entitledSilicon Rich Insulators. According to various embodiments, the chargetrapping layer includes nitride, oxynitrides, metal oxides, metalsilicides, composites such as oxide-nitride or oxide-alumina, orlaminates such as oxide-nitride or oxide-alumina or other material thatfunctions as a charge trapping layer. One of ordinary skill in the artwill understand, upon reading and comprehending this disclosure, thatmany other materials or combination of layers may be selected thatprovide the desired energy barriers, and thus provide the desired chargetrapping characteristics.

As will be described in more detail below, positive charges (holes) aregenerated in the PD-SOI NFET due to impact ionization at the drain edgeand alters the floating body potential. In this embodiment a part ofthese charges are trapped by the charge trapping region 130 (e.g. SRNlayer) in the body region 112. The trapped charges effect the thresholdvoltage (V_(T)), and thus the channel conductance, of the PD-SOI-FET.According to various embodiments, the source current (I_(S)) of thePD-SOI-FET is used to determine if charges are trapped in the bodyregion, and thus is used to determine the logic state of the memorycell.

FIG. 2 illustrates a p-channel one transistor SOI non-volatile memorycell according to various embodiments of the present subject matter. Oneof ordinary skill in the art, upon reading and comprehending thisdisclosure, will understand the structural similarities between thePD-SOI-PFET device and the PD-SOI-NFET device. Some of these structuralsimilarities are not addressed again here for the purpose of simplifyingthe disclosure.

With respect to the illustrated PD-SOI-PFET, the body region 212 isdoped with n− type impurities, and the first and second diffusionregions 214 and 216 are doped with p+ impurities. Negative charges(electrons) are generated in the PD-SOI-PFET at the drain edge andalters the floating body potential. A part of these charges are trappedby the charge trapping region 230 (e.g. SRN layer) in the body region212. The trapped charges affect the threshold voltage (V_(T)), and thusthe channel conductance, of the PD-SOI-PFET in a similar fashion to thePD-SOI-NFET. According to various embodiments, the source current(I_(S)) of the PD-SOI-PFET is used to determine if charges are trappedin the body region, and thus is used to determine the logic state of thememory cell.

In order to simplify this disclosure, memory cells containingPD-SOI-NFET devices are illustrated and described. One of ordinary skillin the art will understand, upon reading and comprehending thisdisclosure, that the present subject matter is not limited toPD-SOI-NFET devices and could be extended to PD-SOI-PFET devices and toboth NFET and PFET FD-SOI devices.

FIG. 3 illustrates a first memory read scheme according to variousembodiments of the present subject matter. In the illustrated system332, the state of the cell 300 is sensed using a direct cell-currentsense amplifier scheme. The memory cell 300 is connected to the currentsense circuitry 334, which is used to sense the source current (I_(S)),and thus the state of the memory cell 300. The schematic of the memorycell illustrates a capacitive coupling between the substrate and thePD-SOI-NFET of the memory cell. As shown in FIG. 1, the BOX layer 106forms a dielectric between the substrate 102 and the body region 112.Aside from the gate-body and body substrate capacitance 333 shown inFIG. 3, an additional series capacitance 335 is associated with thecharge-trapping region. The charge trapping characteristics isillustrated by dotted lines in the capacitor 335.

The direct cell-current sense amplifier scheme can be compared to thesensing schemes associated with static random access memory (SRAM). Oneof ordinary skill in the art will understand, upon reading andcomprehending this disclosure, that the memory cell can be designed andthe performance of the memory cell specified such that the directcell-current sense amplifier scheme can be used.

FIG. 4 illustrates a second memory read scheme according to variousembodiments of the present subject matter. In the illustrated system432, the state of the cell 400 is sensed using a reference cell 436 anda current mode differential sense amplifier scheme. This scheme can becompared to the sensing schemes associated with dynamic random accessmemory (DRAM). Both the memory cell 400 and the reference cell 436 areconnected to the current sense circuitry 434, which is used to comparethe source current (I_(S)) of the memory cell 400 with the current(I_(REF)) of the reference cell 436 to determine the state of the memorycell 400.

FIG. 5 illustrates electrical waveforms associated with reading a memorystate “1” and a memory state “0” according to various embodiments of thepresent subject matter. For the illustrated read operations, a positivegate voltage (V_(G)) and a positive drain voltage (V_(D)) are appliedwhile the substrate voltage is held at a reference voltage (e.g.ground). One of ordinary skill in the art will understand, upon readingand comprehending this disclosure, that the terms positive and negativeare relative terms with respect to the reference voltage.

When the memory cell is in a memory state “1” in which holes are storedin the charge trapping region within the floating body of the PD-SOINFET device, the threshold of the device decreases resulting in a highersource current (I_(S)), represented generally at 538. When the memorycell is in a memory state “0” in which the stored holes are neutralizedin the floating body of the PD-SOI NFET device, the threshold of thedevice increases resulting in a lower source current (I_(S)),represented generally at 540. The difference between the source currentin the memory state “1” can be two to three orders of magnitude greaterthan the source current in the memory state “0”.

Memory Cell Operation

The one transistor SOI non-volatile memory cell of the present subjectexploits the body charging associated with the excess carriers in thebody (also referred to as floating body effect) of PD-SOI devices tostore information. Part of the excess carriers in the floating body getstrapped and stored in the charge trapping layer in the body. Thistrapped stored charge in the transistor body affects the thresholdvoltage (V_(T)). A lower threshold voltage (V_(T)) increases the sourcecurrent (I_(S)) of the transistor, and a higher threshold voltage(V_(T)) decreases the source current (I_(S)). The source current (I_(S))of the memory cell transistor is used to determine the state of thememory cell.

There are a number of ways in which to generate the excess charge in aPD-SOI transistor. A first method for generating charge in PD-SOItransistors involves impact ionization in a field effect transistor(FET) operational mode. A second method for generating charge in PD-SOItransistors involves a relatively low field parasitic bipolar junctiontransistor turn-on mode. These methods for generating charge aredescribed in detail below with respect to a memory operation embodimentfor n-channel FET devices. The excess charge for the NFET devices areholes. One of ordinary skill in the art will understand, upon readingand comprehending this disclosure, how to generate complementary charge(electrons) using the high field impact ionization mode and therelatively low field parasitic bipolar transistor mode for p-channel FETdevices.

FET Mode of Operation

The FET operational mode for generating charges in the body of a PD-SOItransistor involves high field impact ionization at the drain edge ofthe FET device. In various embodiments, the generated positive charge inthe body region of the PD-SOI-NFET device is directed toward the chargetraps in the body region by providing an appropriate electromotive force(EMF) field vertical (or normal) to the FET channel. The EMF field isprovided by applying an appropriate voltage difference between the gateand the substrate.

FIGS. 6A-6D illustrate a write operation for a memory cell in a FET modeof operation according to various embodiments of the present subjectmatter. In the FET operational mode, a high positive drain voltage pulseis applied when the word line is held high such that the transistoroperates in saturation (FIG. 6A). An excess of positive body charge 642is created near the drain region due to the impact ionization mechanismassociated with the device operation in saturation (FIG. 6B). A negativesubstrate voltage pulse is applied (FIG. 6C) in a timely sequence afterthe positive charge is generated by the impact ionization mechanism. Thenegative substrate voltage provides a EMF field across the body regionwhich causes the generated holes 642 to drift toward the charge trappingregion 630 (FIG. 6D). In various embodiments, the charge trapping region630 includes a layer of SRN near the BOX/body interface. In this state,the raised positive body potential lowers the threshold voltage (V_(T))of the transistor.

FIGS. 7A-7B illustrate an erase operation for a memory cell in anNFET-SOI mode of operation according to various embodiments of thepresent subject matter. A negative drain voltage pulse is applied tocreate an excess negative charge in the body. Additionally, a positivesubstrate voltage is applied in a timely sequence. An EMF field 748 isthereby set up from the substrate to the gate to attract the excesselectrons toward the charge trapping region 730 which then neutralizesthe trapped holes in the charge trapping region. The neutralization ofthe previously trapped positive charge lowers the body potential andconsequently raises the threshold voltage (V_(T)) of the transistor.

FIG. 8 illustrates electrical waveforms associated with writing anderasing a memory cell in a FET mode of operation according to variousembodiments of the present subject matter. A write 1 operation for aPD-SOI-NFET device involves generating excess holes and trapping theholes in the trapping layer of the body region of the device. Thepositive gate voltage pulse and the large drain voltage pulse, shownwithin the dotted line 850, causes the PD-SOI-NFET to turn on andoperate in a saturated mode. An excess of positive charges (holes) aregenerated in the PD-SOI-NFET body due to impact ionization at the drainedge. The excess holes generated by impact ionization are directedtoward the charge trapping region due to the EMF field associated withthe large negative substrate voltage pulse sequentially imposed inrelationship of 850 and shown within the dotted line 852. Theapplication of the large negative substrate voltage pulse enhances thespeed of charge trapping, but it is not essential for charge trapping orcell operation and is therefore optional.

According to various embodiments, a write 0 operation, also referred toas an erase operation, for the PD-SOI-NFET device involves neutralizingthe trapped holes with electrons generated in the body region of thedevice. Electrons are generated in the body region by forward biasingthe p−n+ junction using a negative drain pulse and a positive substratepulse, shown within the dotted line 854. The generated electrons drifttoward the charge trapping region, where the electrons neutralize thestored holes. The positive substrate pulse, which is desirable but notessential, extends for a duration longer than the negative drain pulseto provide an EMF field across the body that enhances the drift of thegenerated electrons toward the charge trapping region.

Bipolar Junction Transistor (BJT) Mode of Operation

The lateral parasitic Bipolar Transistor mode for generating charges inthe body of a PD-SOI transistor involves a relatively low fieldmechanism. The n-channel FET device includes a parasitic lateral NPNbipolar junction transistor (BJT). Various voltages are applied to thememory cell to cause the NPN transistor to generate positive charges(holes). In various embodiments, the generated positive charge isdirected toward the charge trapping region in the body region byproviding an appropriate electromotive force (EMF) field across the bodyby applying an appropriate voltage difference between the gate and thesubstrate.

FIGS. 9A-9B illustrate a lateral parasitic bipolar junction transistor(BJT) associated with a FET device in the memory cell according tovarious embodiments of the present subject matter. The PD-SOI-NFETtransistor 910 includes a parasitic NPN transistor 956, as illustratedin FIG. 9A. One of ordinary skill in the art will understand, uponreading and comprehending this disclosure, how to apply the teachingscontained herein to a parasitic lateral PNP transistor in a PD-SOI-PFETtransistor.

FIG. 9B is a schematic diagram of the memory cell of the present subjectmatter, and generally illustrates the parasitic BJT 956 in thePD-SOI-NFET transistor 910. The substrate 902 is capacitively coupledacross the BOX layer 906 to the body region 912 of the NFET transistor,which also functions as the base of the parasitic NPN transistor. Thebody region 912 includes charge trapping region 930, such as an SRNcharge trapping layer, for example. For clarity, the body-substratecapacitor in the embodiment consists of two series capacitors: thetrapping layer capacitor and the BOX capacitor between the body and thesubstrate, as shown.

FIGS. 10A-10D illustrate a write operation for a memory cell in aparasitic BJT mode of operation according to various embodiments of thepresent subject matter. A negative gate pulse is applied, and a negativedrain pulse (having a shorter duration than the gate pulse) is appliedduring the negative gate pulse (FIG. 10A). The gate voltage iscapacitively coupled simultaneously to the source and the body regionwhile forward biasing the p−n+ junction between the body region 1012 andthe drain diffusion region 1014. In this condition, the lateral NPNtransistor action generates excess holes 1057 near the drain region 1014of the PD-SOI-NFET (FIG. 10B). As the gate pulse returns to ground, thesubstrate is pulsed negative (FIG. 10C). This negative substrate pulseprovides an additional vertical drift field 1058 through the body fromthe gate to the substrate (FIG. 10D). The vertical drift field 1058enhances the drift of the generated holes 1057 toward the chargetrapping 1030 in the body of the transistor. The negative substratepulse is desirable but not essential for charge trapping or operation.Thus, the charge trapping region stores at least a portion of the holecharges generated in the body region.

FIGS. 11A-11B illustrate an erase operation for a memory cell in aparasitic BJT mode of operation according to various embodiments of thepresent subject matter. The drain-body diode (n+−p) is forward biased byproviding a negative drain pulse and a positive substrate pulse (FIG.11A). The forward biased diode generates electrons 1146 in the bodyregion (FIG. 11B). The gate is kept at a constant low positive potentialas the substrate pulse is applied. The applied substrate pulse overlapsthe negative drain pulse. The positive substrate voltage creates astronger vertical drift field 1148 to enhance movement of the generatedelectrons 1146 toward the charge traps, which neutralizes the trappedholes in the body region of the PD-SOI-NFET device (FIG. 11B). Thesubstrate pulse is desirable to enhance the speed of operation, but isnot essential for neutralizing the trapped holes or cell operation.

FIG. 12 illustrates electrical waveforms associated with writing anderasing a memory cell in a parasitic BJT mode of operation according tovarious embodiments of the present subject matter. A write 1 operationfor a PD-SOI NFET device involves generating holes and trapping theholes in body region of the device. The negative gate voltage pulse andthe large negative drain voltage pulse, shown within the dotted line1260, causes the parasitic bipolar transistor to generate holes in thebody region of the PD-SOI NFET. It is noted that the negative gatevoltage pulse capacitively couples both the source and the body region,and the body region functions as the base of the parasitic BJTtransistor. The body-drain junction is forward biased because the drainvoltage is more negative than the gate voltage. Near the end of the gatevoltage pulse, a large negative substrate voltage pulse, shown withinthe dotted line 1262, provides an enhanced EMF field that directs thegenerated holes toward the charge trapping region.

A write 0 operation, also referred to as an erase operation, for thePD-SOI NFET device involves neutralizing the trapped holes withelectrons generated in the body region of the device. A small positivevoltage, illustrated by the dotted line 1264, is applied to the gate.Electrons are generated in the body region by forward biasing the p−n+junction using a negative drain pulse and a positive substrate pulse,shown within the dotted line 1266. The electron drift is toward thecharge traps, where the electrons neutralize the stored holes. Thepositive substrate pulse extends for a duration longer than the negativedrain pulse, allowing the substrate pulse and the gate potential toprovide an EMF field that assists the drift of the generated electronstoward the charge centers of the charge trapping region (charge trappinglayer).

The following table provides one example of a BJT mode of operation inwhich Vdd=2.5 V.

BIT WORD OPERATION LINE LINE SUBSTRATE REMARKS Write “1” −2.5 V −1.7 V−2.5 V Holes are   1-5 ns   2-10 ns   2-10 ns generated in the body andare trapped in the trapping layer. V_(T) is reduced by 200 mV. Write “0”−2.5 V   0.8 V   2.5 V Electrons are   1-5 ns   2-10 ns generated in thebody and neutralize the trapped holes. V_(T) returns to original value.Half-Select   0.3 V As above. As above. No change. Cells Read “1”   0.3V   0.8 V Gnd Current is 2-3 orders of magnitude higher. Read “0”   0.3V   0.8 V Gnd Current is lower. Device threshold is designed to put thedevice in sub- threshold operation for a Read “0” operation.Scalability of Memory Cell

According to various embodiments, the memory cell is fully scalable. Thefunctionality of the memory cell is independent of the feature size. Thecell density directly benefits from the reduction in feature size.Additionally, contrary to the characteristics of the conventional DRAMcell, this memory cell improves in functionality and characteristics asthe feature size is reduced due to the following reasons. One reason isthat the device short channel effect improves due to the reduction inthe volume of neutral region of the body and due to the“narrow-width-effect” that raises the “base” threshold of the device.Another reason is that charge trapping efficiency is improved due to theincrease in carrier energy of the excess carriers as the body volume isreduced. The device leakage is also reduced due to both of thesereasons. Additionally, trapped charges extend the body depletionregions, reducing device parasitic capacitance. This further improvesintrinsic device switching speed. Furthermore, the concept is notlimited to PD-SOI FETs, but is extendable to FD-SOI FETs as well.

System Level

FIG. 13 is a simplified block diagram of a high-level organization ofvarious embodiments of a memory device according to various embodimentsof the present subject matter. The illustrated memory device 1368includes a memory array 1370 and read/write control circuitry 1372 toperform operations on the memory array via communication line(s) 1374.

The memory array 1370 includes a number of one transistor SOInon-volatile memory cells 1300 as described above. Although theillustrated memory cells 1300 include PD-SOI NFET devices, the presentsubject matter is not limited to PD-SOI-NFET devices. The memory cellsin the array are arranged in rows and columns. In various embodiments,word lines connect the memory cells in the rows, and bit lines connectthe memory cells in the columns. According to various embodiments, thememory cells in the array are formed in a single substrate. According tovarious embodiments, the substrate for one or more memory cells isisolated from the substrate(s) for other memory cells. Thus, theseembodiments provide the ability to provide different substrate voltagesto different portions of the memory array.

The read/write control circuitry 1372 includes word line select andpower circuitry 1374, which functions to select a desired row and toprovide a desired power signal or pulse to the selected row. Theread/write control circuitry 1372 further includes bit line select andpower circuitry 1376, which functions to select a desired column and toprovide a desired power signal or pulse to the selected column. Theread/write control circuitry 1372 further includes substrate potentialcontrol circuitry 1378 which functions to provide a desired power signalor pulse to the substrate. According to various embodiments in which thememory array includes a number of isolated substrates, the substratepotential control circuitry 1378 also functions to select a desiredsubstrate to which the desired power signal or pulse is applied. Theread/write control circuitry 1372 further includes read circuitry 1380,which functions to detect a memory state for a selected memory cell inthe memory array 1370. According to various embodiments, the readcircuitry 1380 uses a direct cell-current sense amplifier scheme such asthat illustrated in FIG. 3. According to various embodiments, the readcircuitry 1380 uses a reference cell and a current mode differentialsense amplifier scheme such as that illustrated in FIG. 4.

FIG. 14 is a simplified block diagram of a high-level organization ofvarious embodiments of an electronic system according to the presentsubject matter. In various embodiments, the system 1400 is a computersystem, a process control system or other system that employs aprocessor and associated memory. The electronic system 1400 hasfunctional elements, including a processor or arithmetic/logic unit(ALU) 1402, a control unit 1404, a memory device unit 1406 and aninput/output (I/O) device 1408. Generally such an electronic system 1400will have a native set of instructions that specify operations to beperformed on data by the processor 1402 and other interactions betweenthe processor 1402, the memory device unit 1406 and the I/O devices1408. The control unit 1404 coordinates all operations of the processor1402, the memory device 1406 and the I/O devices 1408 by continuouslycycling through a set of operations that cause instructions to befetched from the memory device 1406 and executed. According to variousembodiments, the memory device 1406 includes, but is not limited to,random access memory (RAM) devices, read-only memory (ROM) devices, andperipheral devices such as a floppy disk drive and a compact disk CD-ROMdrive. As one of ordinary skill in the art will understand, upon readingand comprehending this disclosure, any of the illustrated electricalcomponents are capable of being fabricated to include one-transistor,non-volatile SOI memory cells in accordance with the present subjectmatter.

The illustration of the system 1400 is intended to provide a generalunderstanding of one application for the structure and circuitry of thepresent subject matter, and is not intended to serve as a completedescription of all the elements and features of an electronic systemusing one-transistor, SOI non-volatile memory cells according to thepresent subject matter. As one of ordinary skill in the art willunderstand, such an electronic system can be fabricated insingle-package processing units, or even on a single semiconductor chip,in order to reduce the communication time between the processor and thememory device.

Applications containing one-transistor, SOI non-volatile memory cells,as described in this disclosure, include electronic systems for use inmemory modules, device drivers, power modules, communication modems,processor modules, and application-specific modules, and may includemultilayer, multichip modules. Such circuitry can further be asubcomponent of a variety of electronic systems, such as a clock, atelevision, a cell phone, a personal computer, an automobile, anindustrial control system, an aircraft, and others.

Silicon Rich Insulators as Charge Trapping Layer

According to various embodiments of the present subject matter, asilicon-rich-insulator (SRI), such a silicon-rich-nitride (SRN) orsilicon-rich-oxide (SRO), is used to provide charge traps in the bodyregion of PD-SOI-FET devices. In various embodiments, a layer of SRI isformed in the body region near an interface between the body region andthe BOX layer. One of ordinary skill in the art will understand, uponreading and comprehending this disclosure, that FIGS. 15-19 furtherdescribe SRI material.

FIG. 15 is a graph showing refractive index of silicon-rich siliconnitride films versus SiH₂Cl₂/NH₃ flow rate ratio (R). This graph isprovided herein to illustrate the relationship between the siliconamount and the refractive index. The graph indicates that the index ofrefraction increases linearly with increasing silicon content. As such,the index of refraction of the films can be used as an indication of thesilicon content of the films.

FIG. 16 is a graph showing current density versus applied field forsilicon-rich silicon nitride films having different percentages ofexcess silicon. The current density (J) is represented in amperes/cm²,and log J is plotted against the electric field E (volts/cm) for Si₃N₄layers having a SiH₂Cl₂/NH₃ flow rate ratio R of 0.1, 3, 5, 10, 15 and31. This graph is provided herein to illustrate the relationship betweenthe amount of silicon and the conductivity of the film. The plot showsthat the Si₃N₄ layers having small additions of silicon (R=3 and 5)exhibit a relatively small conductivity increase over stoichiometricSi₃N₄. The plot further shows that increasing silicon content at orabove R=10 substantially increases or enhances the conductivity.

FIGS. 17 and 18 provide graphs that illustrate the relationship betweenthe flatband shift and applied fields for films having varyingpercentages of excess silicon as represented by the SiH₂Cl₂/NH₃ flowrate ratio R. FIG. 17 is a graph showing flatband shift versus time atan applied field of 4×10⁶ volts/cm for silicon-rich silicon nitridefilms having varying percentages of excess silicon. For R=3, theflatband shift is greater than the shifts produced by films having an Rof 0.1, 10 or 15. The film having an R of 10 provides a greater flatbandshift than a film having an R of 15. FIG. 18 is a graph showing flatbandshift versus time at an applied field of 7×10⁶ volts/cm for silicon-richsilicon nitride films having varying percentages of excess silicon. Theflatband shift produced by the R=3 film is even greater than that shownin FIG. 17, while the shifts produced by the R=10 and R=15 films do notchange as appreciably. FIGS. 17 and 18 are provided to illustrate thecharacteristics of a charge storing medium and a more conductive chargeinjector medium as further explained below.

The graphs of FIGS. 15-18, which were described above, indicate that atlow additional silicon content, silicon-rich Si₃N₄ films function as acharge storing medium as they exhibit appreciably enhanced trappingcharacteristics (as shown by the high flatband shifts at moderate andhigh applied electric fields in FIGS. 17 and 18, respectively) withoutexhibiting appreciably enhanced conductivity characteristics as shown inFIG. 15.

Silicon-rich silicon nitride films deposited at an R of 3 or 5 (for arefractive index of 2.10 and 2.17, respectively) will possess a chargestoring function or property normally provided by a polysilicon floatinggate of a EEPROM cell. In general, silicon-rich nitride films having anR greater than 0.1 and less than 10 (or, more specifically, having anindex of refraction between approximately 2.10 and 2.30) will provideappreciably enhanced charge trapping or charge storing propertieswithout providing appreciably enhanced charge conduction. This chargetrapping is characteristic of a charge storing medium that can be usedas a charge trapping material in the present subject matter.

Silicon-rich nitride films having an R greater than 10 (or, morespecifically, having an index of refraction greater than 2.3) arereferred to as an injector medium. A silicon-rich Si₃N₄ (SRN) injectorprovides appreciably enhanced charge conductance without providingappreciably enhanced charge trapping over stoichiometric Si₃N₄. This isillustrated in FIGS. 17 and 18, which shows progressively reducedflatband shifts for R=10 and R=15 with progressively increasedconduction.

FIG. 19 is a graph showing apparent dielectric constant K versusrefractive index for both Silicon Rich Nitride (SRN) and Silicon RichOxide (SRO). The SRN and SRO plotted in this graph were provided using aLow Pressure Chemical Vapor Deposition (LPCVD) process. The SRO wasfabricated at approximately 680° C., and the fabricated structureincluded 100 Å oxide and 150 Å SRO. The SRN was fabricated atapproximately 770° C., and the fabricated structure included 45 Å oxideand 80 Å SRN. As shown in the graph, the dielectric constant of siliconis around 12. Materials with a higher K than silicon are conventionallytermed a high K material, and materials with a lower K than silicon areconventionally termed a low K material. SRN that has a refractive indexof 2.5 or greater and SRO that has a refractive index of 1.85 or greaterhave apparent dielectric constants that are greater than 12. InjectorSRI includes these high K SRO and high K SRN. Charge-centered SRIincludes low K SRO and low K SRN.

Memory Cell Fabrication Using Charge Trapping SRI Layer

The processing of the memory cell of the present subject matter involvesstandard processing associated with PD-SOI device fabrication. Thechannel implant is adjusted to appropriately tailor the FET threshold.According to various embodiments, the BOX-body interface includes atrapping layer, such as an SRI layer.

Various embodiments create the trapping layer using the followingprocess. Standard processing steps are performed through the shallowtrench isolation (STI). A block mask is applied to device and open theactive retention of the FET device. In these embodiments, the FET deviceis an NFET device, but the present subject matter is not limited to NFETdevices. Silicon, ammonia (NH₃), and optionally hydrogen are ionimplanted with an appropriate energy and concentration to achieve adesired refractive index after post processing anneal. In variousembodiments, ammonia is replaced by active nitrogen. In variousembodiments silicon is replaced by other active silicon sources such assilane, dichlorosilane, and the like. A post-implant inert anneal isperformed. According to various embodiments, the anneal includes a rapidthermal anneal (RTA). According to various embodiments, the annealincludes an inert plasma anneal in nitrogen. Standard PD-SOI CMOSfabrication steps are capable of being performed thereafter to completethe fabrication of the memory cell.

Silicon Nitride and Oxynitrides as Charge Trapping Layer

In various embodiments, the charge trapping layer includesstoichiometric nitride (Si₃N₄). One of ordinary skill in the art willunderstand, upon reading and comprehending this disclosure, how to forma Si₃N₄ layer at a desired location at or near the box region usingthermal recombination, chemical vapor deposition (CVD), plasma enhancedchemical vapor deposition (PECVD), atomic layer deposition (ALD), aswell as by ion implantation of reactive elements followed by anappropriate anneal. Memory cells can be fabricated using charge trappingSi₃N₄ in a manner similar to that described above for SRI.

In various embodiments, the charge trapping layer includes oxynitrides.A brief description of oxynitride film materials follows sinceoxynitrides can be readily used as charge trapping layers. In variousembodiments, oxynitrides are formed by incorporating nitrogen intostoichiometric silicon dioxide (SiO₂). In various embodiments,oxynitrides are formed by incorporating oxygen into Si₃N₄. In variousembodiments, oxynitrides are formed by incorporating oxygen and nitrogenin silicon. In various embodiments, oxynitrides are formed by combiningappropriate reactive sources of silicon, oxygen and nitrogen using avariety of techniques such as thermal recombination, chemical vapordeposition (CVD), plasma enhanced chemical vapor deposition (PECVD),atomic layer deposition (ALD), ion implantation and the like. In variousembodiments, the nitrogen is implanted directly into the BOX region toform the oxynitride trapping layer. In various embodiments, oxygen andnitrogen or appropriate sources of oxygen and nitrogen are implantedinto the active silicon region above the BOX region to form theoxynitride trapping layer.

The author pioneered the development of oxynitride films during hisinvestigations in 1975-76. Oxynitride films vary in composition inrelative nitrogen to oxygen ratios from the extreme stoichiometric rangeof SiO₂ of zero nitrogen content to the other extreme of stoichiometricnitride of Si₃N₄ of zero oxygen content. A physical property of anoxynitride film, such as the refractive index (ν) of the oxynitridefilm, can be correlated to the concentration of oxygen, nitrogen andsilicon in such a film. Auger analysis of such films are shown in Table1.

TABLE 1 Ref. Concentration (a/o) Calculated Index O N Si Formula n_(N) +n_(O) n_(N)/(n_(N) + n_(O)) 1.91 6.9 49.8 43.0 Si O_(0.161) N_(1.159)1.320 0.88 1.89 12.0 50.4 37.7 Si O_(0.318) N_(1.338) 1.656 0.81 1.8712.6 48.6 38.6 Si O_(0.327) N_(1.261) 1.588 0.79 1.84 20.8 41.0 38.1 SiO_(0.546) N_(1.077) 1.623 0.66 1.74 28.1 32.4 39.4 Si O_(0.714)N_(0.821) 1.535 0.54 1.65 43.7 17.6 38.6 Si O_(1.132) N_(0.457) 1.5890.29 1.60 45.5 19.7 34.8 Si O_(1.305) N_(0.565) 1.870 0.30 I_(Si)/I_(O)ratio (in SiO₂) = 0.169 I_(Si)/I_(N) ratio (in Si₃N₄) = 0.501

The ratio of nitrogen over nitrogen plus oxygen in oxynitride films canbe directly and linearly correlated with the refractive indices of suchfilms. This is shown in FIG. 20, which is a graph showing a linearcorrelation between the ratio of nitrogen over nitrogen plus oxygen inoxynitride films and the refractive index of these films. The entirerange from SiO₂ (refractive index of 1.47) and Si₃N₄ (refractive indexof 2.0) is shown. Chemical properties such as etch rate in 7:1 bufferedHF can also be correlate with the oxynitride films. FIG. 21 is a graphshowing the etch rate of 7:1 buffered HF as a function of refractiveindex. Charge transport and trappings of oxynitride films varysignificantly with composition which can be conveniently characterizedby the refractive indices. FIG. 22 is a graph showing low fieldconduction of silicon-oxynitride films as a function of refractive indexin a relatively low field current range, and FIG. 23 is a graphillustrating current density versus field of Si₃N₄ and Si_(x)O_(y)N_(z)and showing charge transport for the entire filed range for nitride,Si₂ON₂ oxynitride, and oxygen-rich oxynitride.

The author had also investigated charge trapping probability ofoxynitride films as a function of refractive indices. It has been foundthat trapped charge density for electrons and holes peaks in the rangeof compositions between refractive index of 1.8 to 1.9. The range ofcompositions of films covering refractive index between 1.75 to 1.9could be considered for trapping layer for various embodiments of thepresent invention. It was also found that below the refractive index of1.75, charge trapping decreases with increasing oxygen concentration.The trap energy depth, which affects the charge retention, also varieswith the composition of oxynitride. Memory cells can be fabricated usingcharge trapping oxynitride in a manner similar to that described abovefor SRI.

Charge trapping layers that include Si₃N₄ and charge trapping layershave been described above. A benefit of these charge trapping layers isthat these layers can be formed using common and relatively simplesemiconductor processes.

Other Charge Trapping Layers

Although SRI layers are specifically cited as “charge trapping layers,”many other charge trapping materials are used as a charge trappingmedium in many other embodiments. For example, metal oxides (includingtransition-metal-oxides), metal suicides and composites or laminates(such as a combination of silicon oxynitride, oxide nitride oroxide-alumina) can be used to form charge trapping layers. Nano-voidsalso can be used to form charge trapping layers. These examples are notintended to be an exhaustive list of the number of ways to form chargetrapping layers that can be used according to the present subjectmatter. One of ordinary skill in the art will understand that suchlayers are incorporated by appropriate fabrication processes.

Sidewall Charge Trapping Region

FIGS. 24A-24C illustrate a one transistor SOI non-volatile memory cellwith a sidewall charge trapping region according to various embodimentsof the present subject matter. The memory cell embodiment illustrated inFIG. 24A generally corresponds to the memory cells illustrated in FIGS.1 and 2. The memory cell is formed on a substrate 2402, such as asilicon substrate, for example. The memory cell is isolated from thesubstrate 2402 via a buried insulator, such as a buried oxide (BOX)layer 2406, and from other devices via shallow trench isolation (STI)regions 2408. The illustrated transistor includes a floating body region2412, a first diffusion region 2414, and a second diffusion region 2416.A channel region 2418 is formed in the body region 2412 between thefirst and second diffusion regions 2414 and 2416. The illustrated memorycell includes a bit line contact or drain contact connected to the firstdiffusion region 2414, and a source line contact connected to the seconddiffusion region 2416. A gate 2424, such as a polysilicon gate, isseparated from the channel region 2418 by a gate insulator region 2426.The illustrated memory cell includes a word line contact or gate contactconnected to the gate 2424.

The body region 2412 of the illustrated FET device includes at least onecharge trapping region. The illustrated charge trapping region includesa charge trapping layer 2430A formed along or proximate to at least aportion of an interface between the body region 2412 and the BOX layer2406, and a sidewall charge trapping region 2430B formed along orproximate to at least a portion of at least one sidewall of the bodyregion 2412. One of ordinary skill in the art will understand how tofabricate the sidewall charge trapping region along or proximate to abody region sidewall.

The location of the charge trapping region in the body region can bevaried. In various embodiments, the location the charge trapping regionis on or near the BOX-body interface. In other embodiments, the chargetrapping region 130 is located elsewhere in the body region, includingat or near at least a portion of at least one sidewall of the bodyregion. The charge trapping region is positioned such that it will notinterfere with conductance. For example, various embodiments of thepresent subject matter position the charge trapping region along asidewall of the body region at a depth below approximately 50 Å from thesurface. One of ordinary skill in the art will understand, upon readingand comprehending this disclosure, that the charge trapping region canbe designed to provide desired charge trapping characteristics usingvarious materials and techniques.

Charges are generated in the FET due to impact ionization at the drainedge, which alters the floating body potential. In this embodiment apart of these charges are trapped by the charge trapping region in thebody region. The trapped charges effect the threshold voltage (V_(T)),and thus the channel conductance, of the FET. According to variousembodiments, the source current (I_(S)) of the FET is used to determineif charges are trapped in the body region, and thus is used to determinethe logic state of the memory cell.

FIG. 24B illustrates the memory cell along line 24B—24B of FIG. 24A. Aword line (WL) is illustrated over a number of body regions 2412 for anumber of memory cells. The illustrated charge trapping region includesa charge trapping layer 2430A formed along or proximate to at least aportion of an interface between the body region 2412 and the BOX layer2406, and a sidewall charge trapping region 2430B formed along at leasta portion of the illustrated sidewalls of the body region 2412.

FIG. 24C illustrates a top view of the memory cell along line 24C—24C ofFIG. 24A. The illustration includes a word line (WL) intersected by abit line (BL). The illustrated charge trapping region includes asidewall charge trapping region 2430B formed along a first sidewall(illustrated in the figure on the right side of the body region), formedalong a portion of a second sidewall (illustrated on the top of thefigure) and formed along a portion of a third sidewall (illustrated onthe bottom of the figure). Thus, in the embodiment illustrated in FIGS.24A-24C, the sidewall charge trapping regions 2430B form a U-shaperegion surrounding the second diffusion region 2416 (e.g. sourceregion), and the charge trapping layer 2430 forms a floor near thebottom of the body region.

One of ordinary skill in the art will understand, upon reading andcomprehending this disclosure, that the memory cell can be designed withvariously sized, shaped and positioned charge trapping regions. Thus,various embodiments include a sidewall charge trapping region without acharge trapping layer proximate to the BOX interface, variousembodiments include a charge trapping layer proximate to the BOXinterface without a sidewall charge trapping region, and variousembodiments include a sidewall charge trapping region and a chargetrapping layer proximate to the BOX interface. Various embodiments of asidewall charge trapping region include a region along an entiresidewall of the body region, various embodiments of a sidewall chargetrapping region include a region along a vertical and/or horizontalportion of a sidewall of a body region, and various embodiments of asidewall charge trapping region include a region along at least aportion of two or more sidewalls of a body region. Various embodimentsof a sidewall charge trapping region are formed at or proximate to aninterface between the body region and the STI 2408.

Conclusion

The present subject matter relates to non-volatile SOI memory cells. Thepresent subject matter exploits the floating body effect associated withSOI-FET devices. The memory cell includes charge trapping regions in thebody region of a SOI-FET device. Charges generated by the floating bodyeffect are stored in the charge trapping regions to provide a firstmemory state, and the stored charges are neutralized to provide a secondmemory state. The threshold voltage of the SOI-FET is affected by thestored charges. Thus the channel conductance is capable of being used todetermine the state of the memory cell.

The present subject matter is capable of providing non-volatilememories. Memories according to the present subject matter are capableof maintaining data integrity for up to ten years without refresh.Additionally, the present subject matter is capable of providingnon-volatile memories that can be written using the power supplyvoltage. Thus, the present subject matter does not require thecomplicated circuitry to generate and deliver 4 to 8 times the powersupply voltage such as is required by Flash, EEPROM and the like.Additionally, the present subject matter is capable of providingmemories with an effectively unlimited number of write-erase cyclesduring the system lifetime (10¹³ to 10¹⁴ write-erase cycles in 10years). Additionally, the present subject matter is capable of providingmemories that have fast read and write operations on the order ofnanoseconds rather than milliseconds. Additionally, the present subjectmatter is capable of providing dense memories (4F²).

Previously, a specific memory type (DRAM, SRAM, ROM, Flash, and thelike) was used in specific applications to provide the desired memorycharacteristics for the specific applications. One of ordinary skill inthe art will appreciate, upon reading and comprehending this disclosure,that in view of the above-identified capabilities in a single memorytype, the present subject matter is capable of providing the desirablememory characteristics for an wide range of applications. Thus, thememory for systems that have a number of specific memory applicationscan be economically fabricated according to the present subject matter.

This disclosure includes several processes, circuit diagrams, and cellstructures. The present subject matter is not limited to a particularprocess order or logical arrangement. Although specific embodiments havebeen illustrated and described herein, it will be appreciated by thoseof ordinary skill in the art that any arrangement which is calculated toachieve the same purpose may be substituted for the specific embodimentsshown. This application is intended to cover adaptations or variationsof the present subject matter. It is to be understood that the abovedescription is intended to be illustrative, and not restrictive.Combinations of the above embodiments, and other embodiments, will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the present subject matter should bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

1. A semiconductor structure, comprising: a substrate; a buriedinsulator layer positioned over at least a portion of the substrate; abody region positioned over the buried insulator layer, the body regionincluding a charge trapping region; a first diffusion region and asecond diffusion region to provide a channel region in the body regionbetween the first diffusion region and the second diffusion region; agate insulator layer formed over the channel region; and a gate formedover the gate insulator layer.
 2. The structure of claim 1, wherein thecharge trapping region includes silicon nitride (Si₃N₄).
 3. Thestructure of claim 1, wherein the charge trapping region includes anoxynitride.
 4. The structure of claim 1, wherein the charge trappingregion includes a metal oxide.
 5. The structure of claim 4, wherein themetal oxide includes a transition metal oxide.
 6. The structure of claim1, wherein the charge trapping region includes a metal silicide.
 7. Thestructure of claim 1, wherein the charge trapping region includes acharge trapping layer located at or near an interface between the bodyregion and the buried insulator layer.
 8. The structure of claim 1,wherein the body region has a number of sidewalls, and the chargetrapping region includes a sidewall charge trapping layer located at ornear at least a portion of at least one of the number of sidewalls.
 9. Asemiconductor structure, comprising: a substrate; a buried insulatorlayer positioned over at least a portion of the substrate; a body regionpositioned over the buried insulator layer; a charge trapping regionformed at an interface between the buried insulator layer and the bodyregion; a first diffusion region and a second diffusion region toprovide a channel region in the body region between the first diffusionregion and the second diffusion region; a gate insulator layer formedover the channel region; and a gate formed over the gate insulatorlayer.
 10. The structure of claim 9, wherein the charge trapping regionincludes silicon nitride (Si₃N₄).
 11. The structure of claim 9, whereinthe charge trapping region includes an oxynitride.
 12. The structure ofclaim 11, wherein the oxynitride has a refractive index within a rangeof approximately 1.75 to approximately 1.9.
 13. The structure of claim9, wherein the body region has a number of sidewalls, and the chargetrapping region is further formed at or near at least a portion of atleast one of the number of sidewalls.
 14. A semiconductor structure,comprising: a substrate; a buried insulator layer positioned over atleast a portion of the substrate; a body region positioned over theburied insulator layer; a charge trapping region formed in the bodyregion near an interface between the buried insulator layer and the bodyregion; a first diffusion region and a second diffusion region toprovide a channel region in the body region between the first diffusionregion and the second diffusion region; a gate insulator layer formedover the channel region; and a gate formed over the gate insulatorlayer.
 15. The structure of claim 14, wherein the charge trapping regionincludes silicon nitride (Si₃N₄).
 16. The structure of claim 14, whereinthe charge trapping region includes an oxynitride.
 17. The structure ofclaim 16, wherein the oxynitride has a refractive index within a rangeof approximately 1.75 to approximately 1.9.
 18. The structure of claim14, wherein the body region has a number of sidewalls, and the chargetrapping region is further formed at or near at least a portion of atleast one of the number of sidewalls.
 19. A semiconductor structure,comprising: a substrate; a buried insulator layer positioned over atleast a portion of the substrate; a body region positioned over theburied insulator layer, the body region including a charge trappingoxynitride region, the oxynitride region having a refractive indexwithin a range of approximately 1.75 to approximately 1.9; a firstdiffusion region and a second diffusion region to provide a channelregion in the body region between the first diffusion region and thesecond diffusion region; a gate insulator layer formed over the channelregion; and a gate formed over the gate insulator layer.
 20. Thestructure of claim 19, wherein the oxynitride region is positioned overan interface between the body region and the buried insulator layer. 21.The structure of claim 19, wherein the oxynitride region is positionedat an interface between the body region and the buried insulator layer.22. A semiconductor structure, comprising: a substrate; a buriedinsulator layer positioned over at least a portion of the substrate; abody region positioned over the buried insulator layer, the body regionincluding a charge trapping composite; a first diffusion region and asecond diffusion region to provide a channel region in the body regionbetween the first diffusion region and the second diffusion region; agate insulator layer formed over the channel region; and a gate formedover the gate insulator layer.
 23. The structure of claim 22, whereinthe charge trapping composite includes oxide-nitride.
 24. The structureof claim 22, wherein the charge trapping composite includesoxide-alumina.
 25. The structure of claim 22, wherein the chargetrapping region includes a charge trapping layer located at or near aninterface between the body region and the buried insulator layer. 26.The structure of claim 22, wherein the body region has a number ofsidewalls, and the charge trapping region includes a sidewall chargetrapping layer located at or near at least a portion of at least one ofthe number of sidewalls.
 27. A semiconductor structure, comprising: asubstrate; a buried insulator layer positioned over at least a portionof the substrate; a body region positioned over the buried insulatorlayer, the body region including a charge trapping laminate; a firstdiffusion region and a second diffusion region to provide a channelregion in the body region between the first diffusion region and thesecond diffusion region; a gate insulator layer formed over the channelregion; and a gate formed over the gate insulator layer.
 28. Thestructure of claim 27, wherein the charge trapping laminate includesoxide-nitride.
 29. The structure of claim 27, wherein the chargetrapping laminate includes oxide-alumina.
 30. The structure of claim 27,wherein the charge trapping region includes a charge trapping layerlocated at or near an interface between the body region and the buriedinsulator layer.
 31. The structure of claim 27, wherein the body regionhas a number of sidewalls, and the charge trapping region includes asidewall charge trapping layer located at or near at least a portion ofat least one of the number of sidewalls.
 32. A memory cell, comprising:a substrate; a buried insulator layer formed on the substrate; and atransistor formed on the buried insulator layer, including: a floatingbody region, including a charge trapping region within the floating bodyregion, the charge trapping region including silicon nitride (Si₃N₄); afirst diffusion region and a second diffusion region to provide achannel region in the body region between the first diffusion region andthe second diffusion region; a gate insulator layer formed over thechannel region; and a gate formed over the gate insulator layer.
 33. Thememory cell of claim 32, wherein the transistor is a partially-depletedSOI field effect transistor (PD-SOI-FET).
 34. The memory cell of claim32, wherein the transistor is a fully-depleted SOI field effecttransistor (FD-SOI-FET).
 35. A memory cell, comprising: a substrate; aburied insulator layer formed on the substrate; and a transistor formedon the buried insulator layer, including: a floating body region,including a charge trapping region within the floating body region, thecharge trapping region including an oxynitride; a first diffusion regionand a second diffusion region to provide a channel region in the bodyregion between the first diffusion region and the second diffusionregion; a gate insulator layer formed over the channel region; and agate formed over the gate insulator layer.
 36. The memory cell of claim35, wherein the transistor is a partially-depleted SOI field effecttransistor (PD-SOI-FET).
 37. The memory cell of claim 35, wherein thetransistor is a fully-depleted SOI field effect transistor (FD-SOI-FET).38. A memory cell, comprising: a substrate; a buried insulator layerformed on the substrate; and a transistor formed on the buried insulatorlayer, including: a floating body region, including a charge trappingregion within the floating body region, the charge trapping regionincluding a metal oxide; a first diffusion region and a second diffusionregion to provide a channel region in the body region between the firstdiffusion region and the second diffusion region; a gate insulator layerformed over the channel region; and a gate formed over the gateinsulator layer.
 39. The memory cell of claim 38, wherein the metaloxide includes a transition metal oxide.
 40. The memory cell of claim38, wherein the transistor is a partially-depleted SOI field effecttransistor (PD-SOI-FET).
 41. The memory cell of claim 38, wherein thetransistor is a fully-depleted SOI field effect transistor (FD-SOI-FET).42. A memory cell, comprising: a substrate; a buried insulator layerformed on the substrate; and a transistor formed on the buried insulatorlayer, including: a floating body region, including a charge trappingregion within the floating body region, the charge trapping regionincluding a metal silicide; a first diffusion region and a seconddiffusion region to provide a channel region in the body region betweenthe first diffusion region and the second diffusion region; a gateinsulator layer formed over the channel region; and a gate formed overthe gate insulator layer.
 43. The memory cell of claim 42, wherein thetransistor is a partially-depleted SOI field effect transistor(PD-SOI-FET).
 44. The memory cell of claim 42, wherein the transistor isa fully-depleted SOI field effect transistor (FD-SOI-FET).
 45. A memorycell, comprising: a substrate; a buried insulator layer formed on thesubstrate; and a transistor formed on the buried insulator layer,including: a floating body region, including a charge trapping compositewithin the floating body region; a first diffusion region and a seconddiffusion region to provide a channel region in the body region betweenthe first diffusion region and the second diffusion region; a gateinsulator layer formed over the channel region; and a gate formed overthe gate insulator layer.
 46. The memory cell of claim 45, wherein thecharge trapping composite includes oxide-nitride.
 47. The memory cell ofclaim 45, wherein the charge trapping composite includes oxide-alumina.48. The memory cell of claim 45, wherein the transistor is apartially-depleted SOI field effect transistor (PD-SOI-FET).
 49. Thememory cell of claim 45, wherein the transistor is a fully-depleted SOIfield effect transistor (FD-SOI-FET).
 50. A memory cell, comprising: asubstrate; a buried insulator layer formed on the substrate; and atransistor formed on the buried insulator layer, including: a floatingbody region, including a charge trapping laminate within the floatingbody region; a first diffusion region and a second diffusion region toprovide a channel region in the body region between the first diffusionregion and the second diffusion region; a gate insulator layer formedover the channel region; and a gate formed over the gate insulatorlayer.
 51. The memory cell of claim 50, wherein the charge trappinglaminate includes oxide-nitride.
 52. The memory cell of claim 50,wherein the charge trapping laminate includes oxide-alumina.
 53. Thememory cell of claim 50, wherein the transistor is a partially-depletedSOI field effect transistor (PD-SOI-FET).
 54. The memory cell of claim50, wherein the transistor is a fully-depleted SOI field effecttransistor (FD-SOI-FET).
 55. A memory cell, comprising: a substrate; aburied insulator layer formed on the substrate; and a transistor formedon the buried insulator layer, including: a floating body region havinga number of sidewalls, the floating body region including a sidewallcharge trapping region at or near at least a portion of at least one ofthe number of sidewalls; a first diffusion region and a second diffusionregion to provide a channel region in the body region between the firstdiffusion region and the second diffusion region; a gate insulator layerformed over the channel region; and a gate formed over the gateinsulator layer.
 56. The memory cell of claim 55, wherein the floatingbody region further includes a charge trapping layer at or near at leasta portion of an interface between the buried insulator region and thefloating body region.
 57. The memory cell of claim 55, wherein thetransistor is a partially-depleted SOI field effect transistor(PD-SOI-FET).
 58. The memory cell of claim 55, wherein the transistor isa fully-depleted SOI field effect transistor (FD-SOI-FET).
 59. A methodof forming a non-volatile memory cell, comprising: forming a body regionover a buried oxide (BOX) insulator on a substrate; forming a chargetrapping region in the body region; forming a first diffusion region anda second diffusion region to define a channel region in the body region;forming a gate dielectric over the channel region; and forming a gateover the gate dielectric.
 60. The method of claim 59, wherein forming acharge trapping region in the body region includes forming the chargetrapping region at an interface between the BOX insulator and the bodyregion.
 61. The method of claim 59, wherein forming a charge trappingregion in the body region includes forming the charge trapping regionover an interface between the BOX insulator and the body region.
 62. Themethod of claim 59, wherein forming a charge trapping region in the bodyregion includes forming the charge trapping region along a portion of atleast one sidewall of the body region.
 63. The method of claim 59,wherein forming a charge trapping region in the body region includesincorporating nitrogen in the charge trapping region.
 64. The method ofclaim 63, wherein forming a charge trapping region in the body regionfurther includes incorporating oxygen to form an oxynitride region. 65.The method of claim 63, wherein incorporating oxygen to form anoxynitride region includes forming an oxynitride region with arefractive index within a range of approximately 1.75 to 1.9.
 66. Themethod of claim 59, wherein forming a charge trapping region in the bodyregion includes forming a metal oxide.
 67. The method of claim 66,wherein forming a metal oxide includes forming a transition metal oxide.68. The method of claim 59, wherein forming a charge trapping region inthe body region includes forming a metal silicide.
 69. The method ofclaim 59, wherein forming a charge trapping region in the body regionincludes forming an oxide-nitride composite.
 70. The method of claim 59,wherein forming a charge trapping region in the body region includesforming an oxide-nitride laminate.
 71. The method of claim 59, whereinforming a charge trapping region in the body region includes forming anoxide-alumina composite.
 72. The method of claim 59, wherein forming acharge trapping region in the body region includes forming anoxide-alumina laminate.
 73. A method of forming a non-volatile memorycell, comprising: forming a body region over a buried oxide (BOX)insulator on a substrate; forming a charge trapping silicon nitride(Si₃N₄) region in the body region; forming a first diffusion region anda second diffusion region to define a channel region in the body region;forming a gate dielectric over the channel region; and forming a gateover the gate dielectric.
 74. The method of claim 73, wherein forming asilicon nitride (Si₃N₄) includes a process selected from a groupconsisting of: thermal recombination, chemical vapor deposition (CVD),plasma enhanced chemical vapor deposition (PECVD), atomic layerdeposition (ALD), and ion implantation.
 75. The method of claim 73,wherein forming a silicon nitride (Si₃N₄) includes incorporatingnitrogen into a silicon body region to form the silicon nitride (Si₃N₄).76. A method of forming a non-volatile memory cell, comprising: forminga body region over a buried oxide (BOX) insulator on a substrate;forming a charge trapping oxynitride region in the body region; forminga first diffusion region and a second diffusion region to define achannel region in the body region; forming a gate dielectric over thechannel region; and forming a gate over the gate dielectric.
 77. Themethod of claim 76, wherein forming the oxynitride region includesforming a silicon nitride (Si₃N₄) region, and incorporating oxygen intothe silicon nitride (Si₃N₄) region.
 78. The method of claim 76, whereinforming the oxynitride region includes forming a stoichiometric silicondioxide (SiO₂) region, and incorporating nitrogen into the silicondioxide (SiO₂) region.
 79. The method of claim 76, wherein forming theoxynitride region includes incorporating nitrogen into a silicon dioxide(SiO₂) BOX region.
 80. The method of claim 76, wherein forming a chargetrapping oxynitride region includes forming an oxynitride region with arefractive index within a range of about 1.75 to about 1.9.
 81. Themethod of claim 76, wherein forming a charge trapping oxynitride regioninclude adjusting an oxygen concentration for the oxynitride region toprovide a desired charge trapping probability and a desired charge trapdepth.
 82. The method of claim 76, wherein forming a charge trappingoxynitride region in the body region includes combining appropriatereactive sources of silicon, oxygen and nitrogen.
 83. The method ofclaim 76, wherein forming a charge trapping oxynitride region in thebody region includes incorporating nitrogen and oxygen into silicon.